Developing and testing cancer drugs is an expensive process that typically involves exhaustive in vivo animal experiments. These experiments assist in analyzing how a drug is transported and delivered to its target in a tumor mass. However animal testing is a slow process and raises ethical concerns.
One recent alternative to animal testing involves the use of microfluidic systems to study the response of tumor cells to treatment protocols. Current microfluidic systems are based on two-dimensional platforms and thus inadequate to fully evaluate the response of a three-dimensional tumor mass.
Targeted drug delivery to tumors is an important challenge to be addressed in order to achieve effective cancer treatment without the toxic side effects of anti-cancer drugs. The ultimate objective of targeted drug delivery is to deliver most of the administered drug to the target, while eliminating or minimizing the accumulation of the drug at any non-target sites. Although many novel therapeutic agents have been developed for cancer treatments including chemotherapeutic agents, antiangiogenic agents, immunotoxins, and small interfering RNA (siRNA), their in vivo efficacy is still relatively poor. The recent development of nanotechnology provides a wide variety of nanostructures, whose properties can be tailored for targeted delivery to a certain extent. These nanostructures include liposomes, polymer micelles, dendrimers, drug nanocrystals, magnetic nanoparticles, gold nanoparticles/nanoshells, nanorods, nanotubes, and drug-polymer conjugates (all of which will be collectively referred to as NPs). Research aiming to precisely control the size and surface properties of these NPs to achieve targeted delivery is ongoing.
Currently these NPs are primarily designed based on so-called “passive” and/or “active” targeting strategies, which rely on extravasation and ligand-receptor interactions, respectively. Passive targeting is based on the physiological observation that tumor vasculature is leakier than normal vasculature. Since the tumor vasculature wall has larger pores (ranging from about 400-600 nm up to 2 μm in diameter obtained from xenograft models) compared to the normal case (typically smaller than 20 nm), the NPs (whose size is in between these cutoff pore sizes and whose surface is PEGylated for prolonged blood circulation) are believed to selectively accumulate at the tumor. The drug accumulation by the difference in this vascular permeability is often called the enhanced permeation and retention (EPR) effect and has been a key rationale to design NPs for targeted delivery.
Active targeting is a strategy to attach ligands on the surface of NPs so that the NPs selectively bind to the target tumor cells or endothelium. Clearly, active targeting occurs only after passive targeting. These strategies result in the improved accumulation of NPs at the tumor, but the in vivo efficacy of NPs and NP-mediated drugs is still significantly impaired. Only about 5% of the administered dose ends up at the target tumors. The remaining significant portion of the NPs is taken up by the reticulo-endothelial system (RES) of the spleen, liver, and lungs. In order to precisely control the transport of the majority of the administered NPs to target tumors, a new paradigm is needed that considers the complexity of their transport processes in vivo beyond the EPR effect.
One of the critical bottlenecks to developing new targeted delivery strategies is a limited quantitative understanding of the in vivo transport behavior of NPs due to a lack of versatile models to systematically study the in vivo transport characteristics. After being administered to a patient's blood stream, the NPs are thought to experience multiple levels of complex transport processes to reach the cancer cells. These processes include blood flow driven transport of the NPs, NP/endothelium interactions, extravasation, interstitial transport and cellular uptake. Because of the leaky vasculature of the tumor, as illustrated in FIG. 1, the NPs are thought to extravasate more in tumor vasculature than in normal vasculature. At the same time, however, the increased interstitial fluid, less functional lymphatic vessels, dense ECM microstructure and high cell packing density of the tumor may result in significantly elevated IFP, which can adversely affect the extravasation and interstitial transport of the NPs. In addition to the elevated IFP, the dense ECM microstructure and high cell packing density can also impair the interstitial transport of the NPs. FIG. 1 illustrates the vascular and tissue structure relevant to fluid and NP transport of normal and tumor tissues. In normal tissue, the endothelium is tightly packed so that the cutoff pore size is small and very low interstitial fluid flow presents. This fluid flows to the lymphatics through the normal ECM, and the IFP minimally builds up. On the other hand, the endothelium of tumor tissue is leaky and has large pores, which leads to high interstitial fluid flow and more extravasation of the NPs. In conjunction with less functional lymphatics and the dense ECM, this increased interstitial fluid flow results in elevated IFP, which adversely affects the extravasation. The compounding effects of the elevated IFP, leaky vasculature and poor vascularization of the tumor are still unknown.
These tumor micro-environmental parameters are highly dynamic, interconnected and vary spatiotemporally [24, 25], and the compounding effects of all these physiological parameters on NP transport are not yet fully understood. The conventional static in vitro systems described above, including cell suspensions and cell monolayers, lack dynamic interactions of tumor micro-environments among the fluids, ECM, cells and NPs, and are therefore inadequate to fully study these complex in vivo transport processes. Xenograft models have been valuable platforms to characterize the in vivo behavior of the NPs. However, even xenograft models often fail to simulate human in vivo environments or to provide a mechanistic explanation of the in vivo behavior of NPs. This are due to: (i) the unknown scaling factors to extrapolate from animal models to human subjects; (ii) the mismatch between human cancer cells and mice matrix environments; (iii) the difficulties to simulate the heterogeneity of tumor micro-environmental parameters; and (iv) the inability to independently control these parameters in the model. Thus, a new model system is greatly desired, in which the tumor micro-environmental parameters can be systematically and independently controlled, but at the same time the dynamic interactions among the fluids, ECM, cells and NPs are maintained
Therefore, there is a need to develop a novel platform to simulate a three-dimensional tumor vasculature system which imitates the complex transport processes inside a tumor, such as transvascular transport, interstitial transport, and cellular transmembrane transport. It would be highly desirable to be able to simulate these processes on a single device, improving repeatability and speed, while reducing use of animals in drug discovery. This novel platform can be used to improve delivery efficacy, particularly for NPs, and to reduce non-specific accumulation at non-targeted sites.